Standard & Poor's said it expects to be the target of a U.S. Department of Justice civil lawsuit over its mortgage bond ratings, the first federal enforcement action against a credit rating agency over alleged illegal behavior tied to the recent financial crisis.

Shares of McGraw-Hill Cos, the parent of S&P, plunged 13.8 percent on Monday after news of the expected lawsuit surfaced, their biggest one-day percentage decline since the 1987 stock market crash, according to Reuters data.

An announcement of a lawsuit is expected on Tuesday, a person familiar with the matter said.

The news also caused shares of Moody's Corp, whose Moody's Investors Service unit is S&P's main rival, to slide 10.7 percent.

It is unclear why regulators may be now focusing on S&P rather than Moody's or Fimalac SA's Fitch Ratings.

"This lawsuit is significant because it could augur future government action or, even worse for the agencies, more litigation by investors," said Jeffrey Manns, a law professor at George Washington University in Washington, D.C.

A civil case involves a lower burden of proof than a criminal case would, and could make it easier for investigators to uncover potential "smoking guns" through subpoenas, he added.

NO MERIT TO LAWSUIT, S&P SAYS

S&P said the expected Justice Department lawsuit focuses on its ratings in 2007 of various U.S. collateralized debt obligations.

The agency had previously disclosed a probe by the U.S. Securities and Exchange Commission into its ratings for a $1.6 billion CDO known as Delphinus CDO 2007-1. It was not immediately clear whether that CDO is a focus of the case.

"A DOJ lawsuit would be entirely without factual or legal merit," S&P said in a statement. "The DOJ would be wrong in contending that S&P ratings were motivated by commercial considerations and not issued in good faith."

In a variety of lawsuits brought by investors, S&P has maintained that its ratings constitute opinions protected by the free speech clause of the U.S. Constitution.